An electric machine converts electrical power into mechanical force and motion. Electric machines are found in numerous applications including household appliances such as fans, refrigerators, and washing machines. Electric drives are also increasingly used in electric and hybrid-electric vehicles.
A rotary electric machine generally has an internal rotating magnet, called the rotor, which revolves inside a stationary stator. The interaction between the rotor electromagnetic field with the field created by the stator winding creates the machine torque. The rotor may be a permanent magnet or it may be an electromagnet. However, if the rotor has permanent magnets embedded therein (i.e., the permanent magnets are not in the rotor surface), the electric machine may be referred to as an interior permanent magnet (IPM) machine.
The part of the machine across which the input voltage is supplied is called the “armature”. Depending upon the design of the machine, either the rotor or the stator can serve as the armature. In an IPM machine, the armature is the stator, and is a set of winding coils powered by input voltage to drive the electric machine.
The reverse task of converting mechanical energy into electrical energy is accomplished by a generator or dynamo. An electrical machine as discussed above may also function as a generator since the components are the same. When the machine/generator is driven by mechanical torque, electricity is output. Traction machines used on hybrid and electric vehicles or locomotives often perform both tasks.
Typically as an electric machine accelerates, the armature (and hence field) current reduces in order to maintain voltage of the stator within limits. The reduction in field current which reduces magnetic flux inside the machine is also called flux or field weakening current. Field weakening current control techniques can be used to increase performance in the torque-speed characteristic of the machine. To retain control of stator current, the machine field may be reduced by a field weakening current control loop. The field or flux weakening in an IPM machine can be accomplished by adjusting the stator excitation, for example. Stator excitation in an IPM machine may be controlled by voltage pulse width modulation (PWM) of a voltage source inverter.
Flux weakening techniques have been used in the past where IPM flux is purposely weakened to reduce the problems associated with high flux, such as over voltage due to high Back-Electro Motive Force (Back-EMF). For example, during a constant torque region of operation of an electric machine, closed loop current regulator control has been used to control the applied PWM voltage excitation so that the instantaneous phase currents follow their commanded values. However, saturation of the current regulators may occur at higher speeds when the machine terminal voltage approaches the maximum voltage of the PWM inverter. Beyond this point, the flux should be weakened to maintain proper current regulation up to the maximum available machine speed. Reducing the magnetic flux inside the machine provides improved power characteristics of the IPM machine at high speeds.
In many applications, the correct current inputs to efficiently weaken the flux are preprogrammed into the flux control circuits of an IPM system. The pre-programming is stored in a data structure such as a data table. Unfortunately time varying rotor temperature changes cause deviation in the flux produced by the preprogrammed flux weakening currents, thus rendering the pre-programmed flux weakening currents to be sub-optimal for the actual flux being produced.
Accordingly, it is desirable to compensate for time varying temperature effects on rotor flux. In addition it is desirable to adjusting stator current in real time for rotor temperature changes. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.